GENERAL INTRODUCTION: POXVIRIDAE

Introduction

Camelpox is a systemic infection of dromedary camels (Camelus dromedarius) characterized by fever and the development of papules, vesicles, pustules and scabs in the skin particularly of the nostrils, lips and eyelids, caused by a species of orthopoxvirus. It has not been reported in bactrian camels (Camelus bactrianus). The camelpox virus is specific to dromedary camels and is found throughout the camel’s distribution in Africa and Asia,^{9, 25, 30, 36} but is absent from the wild dromedary population of Australia. The disease causes major production losses to camel producers.

Both bactrian and dromedary camels are susceptible to infection with a parapoxvirus which causes a localized infection only with lesions similar to those of orf in sheep and goats known as camel ecthyma.

Camelpox is classified within Risk Group 2 for human infection and requires appropriate biosafety and biosecurity handling. Although camelpox is considered as presenting limited public health risk, it is likely that more human cases of camelpox virus will occur due to reduced immunity to orthopoxvirus following the eradication of smallpox.³²

Aetiology

Camelpox belongs to the genus Orthopoxvirus in the family Poxviridae (OPV), subfamily Chordopoxvirinae (see General Introduction: Poxviridae). The viral genome contains 211 putative genes and is 205 719 base pairs long. It contains a unique region of approximately 3 kb, which encodes three ORFs (CMLV185, CMLV186, CMLV187) absent in other OPVs.¹ Phylogenetic analysis indicates that camelpox virus is clearly distinct but most closely related to variola virus (smallpox).¹⁸

There was considerable concern towards the end of the human smallpox eradication programme due to the fact that camelpox virus appeared indistinguishable from smallpox virus, and may cause disease in humans.⁴ However, it was subsequently shown to produce a different cytopathic effect in HeLa and certain other cell lines in which it generated the formation of multinucleate giant cells, unlike smallpox virus.⁵

When grown on the chorioallantoic membrane of embryonated chicken eggs at 37 °C, the lesions it induces are dense white pocks, very similar in appearance to those produced by smallpox virus. However, when cultivated at 34.5 °C the lesions can be distinguished from those of smallpox virus in that pocks with a haemorrhagic centre are produced. Camelpox virus also grows in Vero-, BHK-, lamb testis-, and lamb and calf kidney cells.

Camelpox virus haemagglutinates cockerel erythrocytes.¹⁰

Isolates of camelpox virus were previously identified using restriction endonucleases, and shown to be different from other species of orthopoxvirus.¹⁵ Today, full or partial genome sequencing is used to identify differences between camelpox viruses.¹⁶ More work is required to determine whether differences observed in the genomes of different camelpox virus isolates are responsible for differences in virulence.

Under the electron microscope camelpox virus is morphologically typical of other orthopoxviruses with a characteristic brick shape and distinct from ovoid parapoxviruses.

Epidemiology

The dromedary camel is found in Kenya, Somalia, Ethiopia and the Saharan countries, throughout the Arabian Peninsula, Iraq, Syria, Turkey, Iran, Afghanistan, Pakistan, southern countries of the former USSR, north India, west China and Mongolia. Seroprevalence rates of camelpox virus are high in most regions where the infection is present. In Morocco, seroprevalence rates were between 23-42 per cent in 2003 and 37-56 per cent in 2009 depending on the geographic region and higher in regions with a higher camel population and illegal trans-border animal movement.³³

In endemic situations camelpox is a disease of young camels, affecting them as they lose their colostrum-derived immunity. It possesses many of the epidemiological characteristics of smallpox and sheep- and goatpox, depending for its survival on a constant supply of susceptible camels. It has been thought that infection probably induces life-long sterile immunity, there being no carrier state, but more recently the virus has been identified in atypical skin lesions of camels that suffered from the typical camelpox 12 months previously. Naïve camels that were in contact with camels presenting atypical minute pox-like skin lesions became infected within two weeks, raising the possibility of virus persistence or a carrier in the herd.³⁹ Since atypical pox-like skin lesions could easily be missed by veterinarians and camel handlers, it is possible that these lesions could be important for the maintenance of camelpox virus. More investigative work is required on this issue.

 A reservoir host other than camels is unlikely to exist. It would seem that the virus survives mainly by constant transmission from infected to susceptible animals, and therefore requires a certain minimum size of susceptible population. The size of this population depends on the susceptibility of the host population and, ultimately, on Ro (the basic reproductive number), i.e. the number of susceptible animals infected, on average, by a single diseased animal. Where a population of camels is unable to...